Human B cell activation in vitro: augmentation and suppression by monocytes of the immunoglobulin production induced by various B cell stimulants.

Monocytes and macrophages are able to exert both positive and negative effects on lymphocyte functions. We have studied the role of monocytes in immunoglobulin (Ig) production in vitro by normal human blood lymphocytes, stimulated by polyclonal B cell activators. Mononuclear leukocytes were isolated on Ficoll-Hypaque and separated into monocyte-enriched and monocyte-depleted fractions by differential adherence to Sephadex G-10 or conditioned flasks or by centrifugation on continuous Percoll gradients. These populations and their mixtures were cultured with different activators, including pokeweed mitogen, staphylococcal phage lysate, streptolysin O, and nocardia water soluble mitogen. After 7 to 12 days, the amounts of Ig produced were determined by radioimmunoassay of the culture super-natants. Although even radical removal of monocytes did not completely abrogate Ig production, the presence of at least some monocytes was required for optimal responses; this held in particular for the production of IgG. Nocardia mitogen differed from the other activators in that optimal amounts of Ig were produced after monocyte depletion. When purified monocytes were added in excess to monocyte-depleted lymphocytes or to unseparated cells, a suppression of Ig production occurred. This was observed with all the above B cell activators, and for Epstein-Barr virus (EBV)-induced Ig production as well. When approximately 35% of monocytes were present, Ig production was almost completely inhibited. This phenomenon was not due to digestion of Ig or to cytotoxic effects of the monocytes, and it did not depend upon the method of monocyte isolation. Further studies were performed, mainly with use of pokeweed mitogen. It was found that the suppression was resistant to ≦ 8000 R of irradiation but was abrogated when the monocytes had been heat-killed. Prostaglandins did not appear to play a major role, since addition of indomethacin to the cultures had no effect. T lymphocytes contaminating the monocyte suspensions or present in the cultures did not participate in the suppression, for it also occurred: 1) when T cell-depleted monocytes were added, 2) when the indicator cultures consisted of purified B cells, stimulated with “T helper factors,” and 3) when EBV, a T-independent activator, was used. The latter point also argues against induction of suppression by the activators stimulating the monocytes in vitro, since monocytes do not bind EBV. The outcome of kinetic experiments suggests that the suppression is effected during the first 3 days, when almost no Ig production is demonstrable. We conclude that monocytes can have both augmenting and strongly suppressive effects on Ig production by human B lymphocytes stimulated by polyclonal activators in vitro. This suppression may directly affect B cell activation.